1. Field of the Invention
This invention relates to the field of rotary vane hydraulic pumps especially those for use in an automotive power steering system. The invention pertains particularly to a technique for controlling the flow rate of the pump through a variable orifice and a constant orifice.
2. Description of the Prior Art
A constant displacement rotary vane pump of the type used for power steering systems has a flow rate proportional to rotor speed. The steering gear supplied with pressurized hydraulic fluid from the pump requires high flow rates when vehicle speed is low and low flow rates when vehicle speed is high. A conventional power steering system therefore includes a flow control valve, which is opened to maximize fluid flow to the steering gear when system pressure is low and is closed to bypass flow from the steering gear to the pump inlet when pump rotor speed is high.
Various techniques for controlling operation of the flow control valve have been developed. For example, U.S. Pat. No. 4,289,454 describes a vane pump having two outlet ports, one port being closed after the flow rate exceeds a predetermined magnitude due to an increase in speed of the rotor. The excess fluid normally passing through one of the outlet ports is returned to the pump inlet to increase the fluid flow rate to the steering gear during high speed conditions.
U.S. Pat. No. 4,470,762 describes a pump having a control that bypasses flow from the pump between a cam ring and thrust plate. A spring opens the bypass passage and a pressure plate closes the bypass passage when system pressure rises. The pump control described in U.S. Pat. No. 4,470,764 includes a spring operating on a valve spool to open bypass flow and biased by system pressure to reduce bypass flow. In the vane pump of U.S. Pat. No. 4,470,765, output flow is partially bypassed through a flow control valve. The valve is operated by system pressure to close bypass passages as system pressure rises, thereby increasing flow to the power steering system.
More recently, power steering systems include electronically variable orifices that are opened and closed in response to vehicle speed and steering wheel speed so that the flow rate to the steering gear from the pump outlet is high when the required steering assist is high, particularly at low vehicle speed, and is low when the required steering assist is low, particularly at high vehicle speed and low steering wheel speed. An example of a power steering system controlled in this way is described in U.S. Pat. No. 4,473,128 in which a bypass valve directs a portion of the fluid flow from the pump from the steering gear in response to vehicle speed and angular velocity of the steering wheel. The position of the bypass valve is controlled by a solenoid, energized and deenergized on the basis of control algorithms executed by a microprocessor. The flow control valve described in U.S. Pat. No. 4,691,619 is also operated by a solenoid, which is energized and deenergized in response to vehicle speed. A pressure modulated slide valve is hydraulically piloted by a solenoid-operated valve. Fluid flow to the steering gear is controlled entirely hydraulically in response to vehicle speed and demand requirements represented by the steering gear input.
U.S. Pat. No. 4,485,883 describes a power steering system having a bypass valve controlling the flow rate of fluid directed from the pump outlet to the pump inlet and a constant flow valve for regulating the flow of bypass fluid. This control system reduced the flow rate to the steering gear during steering maneuvers at high speed and increases the flow rate at low speed and during parking maneuvers.
A similar object is realized with the power steering systems described in U.S. Pat. No. 4,561,521; 4,570,735. A vehicle speed sensitive valve operates to deactivate a conventional flow control bypass valve by eliminating differential force on the flow control valve at speeds greater than a predetermined value. U.S. Pat. No. 4,714,413 describes a power steering system of this type. Another control system of this type employing a solenoid-operated vehicle speed sensitive valve in combination with a conventional flow control bypass valve is described in U.S. Pat. No. 4,609,331.